While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser, especially an ArF excimer laser featuring a shorter wavelength. There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemical amplification type resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemical amplification type resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
On use of the chemical amplification type, positive working, resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemical amplification type, positive working, resist compositions adapted for KrF excimer lasers generally use a phenolic resin, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Iodonium salts, sulfonium salts, and bissulfonyldiazomethane compounds are typically used as the photoacid generator. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
Bissulfonyldiazomethanes, especially bisarylsufonyldiazomethanes such as bis(2,4-dimethylphenylsulfonyl)diazomethane and bisphenylsulfonyldiazomethane are advantageously used as the photoacid generator in chemical amplification type resist compositions, especially chemical amplification type, positive working, resist compositions adapted for KrF excimer lasers because they provide a high sensitivity and resolution and eliminate poor compatibility with resins and poor solubility in resist solvents as found with the sulfonium and iodonium salt photoacid generators.
The characteristics required for the photoacid generator differ depending on a particular application of the resist. Preference is given to either a resist composition having an increased transmittance using a photoacid generator having reduced light absorption or a resist composition having a transmittance controlled relatively low using a photoacid generator having noticeable light absorption.
The latter is used in the case of a resist to be used on a high reflectivity substrate where a photoacid generator having light absorption is intentionally added to the resist to reduce the transmittance of the resist film to reduce the influence of reflected light from the substrate, or in the case of a resist film which is so thin that the resist transmittance is unlikely to have an influence.
Naphthyl group is known as the substituent group having strong absorption in proximity to 248 nm. Photoacid generators in the form of sulfonium salts and sulfonyldiazomethanes having naphthyl groups introduced therein have been reported. For example, JP-A 3-103854 discloses a bilaterally symmetric bis(2-naphthalenesulfonyl)diazomethane though no reference is made to its application and characteristic light absorption.
However, these photoacid generators having naphthyl groups are highly crystalline and have poor solubility in resist solvents and poor affinity (solubility) to developers. They will precipitate out, rather than being kept dissolved, during resist preparation and shelf storage or be left on the substrate as insoluble matter (consisting of the photoacid generator or a mixture thereof with the resin) upon development and/or resist film removal.
In JP-A 2000-171964, bilaterally symmetric naphthylsulfonyldiazomethanes, especially phenylsulfonylnaphthylsulfonyldiazomethane and cyclohexylsulfonylnaphthylsulfonyldiazomethane are proposed for the purpose of achieving a high sensitivity. As long as we confirmed, these compounds have poor solubility and are often left on the substrate as insoluble matter (consisting of the photoacid generator or a mixture thereof with the resin) upon development and/or resist film removal.
Aside from the above-discussed problem of insoluble matter upon development and/or removal, there is also a problem that the pattern profile often changes when the period from exposure to post-exposure baking (PEB) is prolonged, which is known as post-exposure delay (PED). Such changes frequently reveal as a slimming of the line width of unexposed areas in the case of chemical amplification type positive resist compositions using acetal and analogous acid labile groups, and as a thickening of the line width of unexposed areas in the case of chemical amplification type positive resist compositions using tert-butoxycarbonyl (t-BOC) and analogous acid labile groups. Since the period from exposure to PEB is often prolonged for the operational reason, there is a desire to have a stable resist composition which is free from such changes, that is, has PED stability.
The solubility of photosensitive agents or photoacid generators was the problem from the age when quinonediazide photosensitive agents were used in non-chemical amplification type resist materials. Specific considerations include the solubility of photoacid generators in resist solvents, the compatibility of photoacid generators with resins, the solubility (or affinity) in a developer of photo-decomposed products and non-decomposed compound (photoacid generator) after exposure and PEB, and the solubility of the photoacid generator and photo-decomposed products thereof in a remover solvent upon resist removal or peeling. If these factors are poor, there can occur problems including precipitation of the photoacid generator during storage, difficulty of filtration, uneven coating, striation, abnormal resist sensitivity, and foreign matter, left-over and staining on the pattern and in spaces after development.
The photoacid generator in resist material is required to meet a fully high solubility in (or compatibility with) a resist solvent and a resin, good storage stability, non-toxicity, effective coating, a well-defined pattern profile, PED stability, and no foreign matter left during pattern formation after development and upon resist removal. The conventional photoacid generators, especially diazodisulfone photoacid generators do not meet all of these requirements.
As the pattern of integrated circuits becomes finer in these days, a higher resolution is, of course, required, and the problem of foreign matter after development and resist removal becomes more serious.